[ELF] - Linkerscript: reimplemented output sections constrains matching functionality.

Previously filtering that was used worked incorrectly. 
For example for next script it would just remove both sections completely:

SECTIONS { 
. = 0x1000;
.aaa : ONLY_IF_RW { *(.aaa.*) } 
. = 0x2000;
.aaa : ONLY_IF_RO { *(.aaa.*) } 
}

Patch fixes above issues and adds testcase showing the issue. Testcase is a subset of
FreeBSD script which has:

.eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) }
...
.eh_frame       : ONLY_IF_RW { KEEP (*(.eh_frame)) }

Differential revision: https://reviews.llvm.org/D23326

llvm-svn: 278486